A spark plug used for an internal-combustion engine, such as an automobile engine, is provided with a center electrode extending, for example, in an axis direction, an insulator disposed outside of the center electrode, a cylindrical metal shell disposed outside of the insulator, and a ground electrode having a base end that is joined to a front end portion of the metal shell. Further, the ground electrode is bent so that a front end portion thereof faces the front end portion of the center electrode. Furthermore, a noble metal tip is provided with the front end portion of the ground electrode in order to improve erosion resistance properties.
The noble metal tip is usually directly joined to the front end portion of the ground electrode by resistance welding. However, in this case, a stress difference is likely to increase due to difference in thermal expansion between a metal material constituting the ground electrode and a metal material constituting the noble metal tip. As a result, a crack is likely to occur in a joint portion between the ground electrode and the noble metal tip, and the noble metal tip is prone to be separated from the ground electrode.
Therefore, there is proposed a technique wherein a relieving layer tip is disposed between the ground electrode and the noble metal tip. The relieving layer tip is made of a metal material having a linear expansion coefficient between that of the metal material constituting the ground electrode and that of the metal material constituting the noble metal tip. See, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-273966. According to this technique, the difference in thermal expansion is made relatively small between the ground electrode and the relieving layer tip, and between the relieving layer tip and the noble metal tip. Consequently, occurrence of the crack in the joint portion can be prevented.